Positive electrode plate for alkaline storage battery and method for producing the same

ABSTRACT

An alkaline storage battery including a strip-shaped porous metal substrate and a material mixture filled into the substrate. The substrate has an unfilled portion where the material mixture is not filled along at least one of two longitudinal sides of the substrate. The substrate has a weight per unit area of 150 to 350 g/m 2 . The material mixture contains an active material and an elastic polymer.

FIELD OF THE INVENTION

The present invention relates to a positive electrode plate for analkaline storage battery and a method for producing the same. Moreparticularly, the invention relates to a positive electrode plate usinga porous metal substrate having three-dimensionally connected pores.

BACKGROUND OF THE INVENTION

Alkaline storage batteries, which can be repeatedly charged anddischarged, have been widely used as power sources for portableequipment. In recent years, in particular, nickel-metal hydride storagebatteries, which have a high energy density and are relativelyenvironmentally friendly, are dominant in the marketplace, and thedemand therefor is rapidly growing in the fields that require high poweroutput such as power tools and hybrid electric vehicles (HEV).

In positive electrode plates for alkaline storage batteries, porousmetal substrates are preferably used as the core material because theyare easy to be filled with a material mixture paste composed of anactive material, and the rolling step after drying of the materialmixture is easily performed. Further, improvement in capacity densitycan be expected. Particularly, foamed nickel substrates, which areproduced by electroplating or electroless plating a urethane sheet withnickel, followed by baking to remove carbon components, are widely usedas porous metal substrates.

High power output alkaline storage batteries have the followingconfiguration to improve current collecting efficiency. On astrip-shaped electrode is formed an unfilled portion, where a materialmixture composed of an active material is not filled, along one side oftwo longitudinal sides thereof. A positive electrode and a negativeelectrode each having this structure are spirally wound with a separatorinterposed therebetween to form a cylindrical electrode group, wherebythe unfilled portion of the positive electrode is positioned at one endof the electrode group and the unfilled portion of the negativeelectrode is positioned at the other end of the same. By welding currentcollector plates thereto, it is possible to efficiently collect currentfrom the electrodes.

Various attempts have been made to develop a method for producing apositive electrode plate for an alkaline storage battery, some of whichare listed below.

(i) A method in which a porous metal substrate in a hoop shape iscontinuously fed into a vessel holding a material mixture paste composedof an active material so as to fill the paste into the substrate, whichis then allowed to pass through a roll smoother to smoothen the surfaceof the substrate filled with the paste, followed by drying and rolling(see Japanese Laid-Open Patent Publication No. Hei 1-163965).

(ii) A method in which a material mixture paste is sprayed to a porousmetal substrate with a high pressure from a nozzle to fill the pasteinto the substrate, which is then allowed to pass through a slit toremove excess paste therefrom, followed by drying and rolling.

(iii) A method in which a material mixture paste is filled from onesurface of a porous metal substrate such that most part of the othersurface is not filled with the paste, followed by drying and rolling. Inthis method, preferably, the material mixture paste is filled from onesurface of the porous metal substrate such that the other surface is notfilled with the paste at all (see Japanese Laid-Open Patent PublicationsNos. Hei 9-106814 and Hei 9-27342).

(iv) A method in which a material mixture paste is sprayed to bothsurfaces of a porous metal substrate in a hoop shape from nozzles eachpositioned close to each surface of the substrate so as to fill thepaste into the substrate while the substrate is moved in thelongitudinal direction thereof. The distance between the nozzle and thesubstrate is set to 1.0 mm or less (see Japanese Laid-Open PatentPublication No. Hei 9-106815).

In the proposals of Japanese Laid-Open Patent Publications Nos. Hei1-163965 and Hei 9-106814, the entire porous metal substrate is filledwith the material mixture paste. However, positive electrode plates foralkaline storage batteries need to have an unfilled portion where amaterial mixture paste is not filled (i.e. exposed portion of asubstrate) to which a current collector plate is welded (see JapaneseLaid-Open Patent Publication No. 2000-113881). Accordingly, it isnecessary to remove the material mixture having been filled in thesubstrate.

In view of this, Japanese Laid-Open Patent Publication No. 2002-75345proposes to press a porous metal substrate filled with a materialmixture such that protrusions (ribs) are formed, to which ultrasonicvibration is applied to remove the material mixture from theprotrusions, after which the protrusions are utilized as unfilledportions to which a current collector plate is welded. This method,however, is accompanied by problems such as more steps, more loss ofactive material and high costs.

Because the amount of a material mixture paste filled in a substratedepends on the porosity of the substrate, it is difficult to fill aconstant amount of material mixture paste into a substrate using theproposals of Japanese Laid-Open Patent Publications Nos. Hei 1-163965and Hei 9-106814, and a variation in the paste filling rate is caused.The paste filling rate is defined by the ratio of the volume of a pastefilled into a substrate to the volume of pores of the substrate.

Further, according to the proposals of Japanese Laid-Open PatentPublications Nos. Hei 1-163965 and Hei 9-106814, because bubbles aregenerated when a paste is filled into a substrate, only a paste fillingrate of about 90 to about 95% can be achieved at most and the porousmetal is exposed on the substrate surface. As a result, metal burrs areeasily formed when electrode plate is cut into a predetermined size.Also, a short circuit is likely to occur due to the exposed metal whenthe positive electrode and a negative electrode are spirally wound witha separator interposed therebetween to form an electrode group. In orderto prevent the above problems, a thick separator should be used, whichmakes it difficult to achieve a high capacity battery.

According to the proposal of Japanese Laid-Open Patent Publication No.Hei 9-27342, it is possible to spray a constant amount of paste from adie, which significantly reduces the variation in paste filling rate inthe longitudinal direction of the substrate. However, this is notpractical because the paste filling rate is very low and therefore theelectrode plates obtained after rolling have different thicknesses. Asfor battery performance, when metal is exposed on the entire one surfaceof an electrode plate, a part of metal is embedded in an adjoiningseparator, which shortens the distance between the positive and negativeelectrodes, resulting in a large amount of self discharge.

The proposal of Japanese Laid-Open Patent Publication No. Hei 9-106815also requires the removal of the material mixture having been filled inthe substrate in order to create an unfilled portion (i.e. exposedportion of a substrate), which results in high costs.

From the viewpoint of increasing productivity, it is proposed to form,in a substrate, an unfilled portion where a material mixture is notfilled by applying a material mixture paste on the substrate in a strippattern. Devices for performing such stripe application are alsoproposed. For example, Japanese Laid-Open Patent Publication No.2000-233151 proposes a device equipped with a means to adjust a slitgap. Such stripe application is effective when a material mixture pasteis applied on a substrate made of a metal foil. However, it is notalways effective because, when a material mixture paste is filled into aporous metal substrate in a stripe pattern, the material mixture pastemay easily enter the unfilled portion.

As mentioned earlier, porous metal substrates such as foamed nickelsubstrates are widely used for positive electrode plates for alkalinestorage batteries. However, conventional porous metal substrates arecostly to produce, and it is difficult to further improve the currentlevel of the filling rate.

Although porous metal substrates having a lower weight per unit areathan conventional ones can be produced at a relatively low cost, the useof a substrate having a low weight per unit area reduces currentcollecting efficiency, leading to a decrease in high rate dischargeperformance and active material utilization rate.

Further, when a material mixture paste is filled into a porous metalsubstrate having a low weight per unit area in a stripe pattern, theentering of the material mixture paste into the unfilled portion isfacilitated, causing a great loss of active material. When a currentcollector is welded to the unfilled portion where the material mixturehas entered, a weld defect may be caused by sparks or the like, reducingthe strength of the welded portion. The material mixture having enteredthe unfilled portion can be removed completely, but it is inefficient.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to provide apositive electrode plate comprising a substrate with a low weight perunit area and having excellent current collecting efficiency. Anotherobject of the present invention is to provide an efficient method forproducing a positive electrode plate by efficiently filling a materialmixture paste into a porous metal substrate having a low weight per unitarea. Still another object of the present invention is to prevent theloss of a material mixture and to avoid a weld defect between anunfilled portion and a current collector plate by achieving a precisedefinition of the interface between a material mixture-filled portionand the unfilled portion.

The present invention relates to a positive electrode plate for analkaline storage battery comprising a strip-shaped porous metalsubstrate and a material mixture filled into the substrate, wherein thesubstrate has an unfilled portion where the material mixture is notfilled along at least one of two longitudinal sides of the substrate,the substrate has a weight per unit area of 150 to 350 g/m², and thematerial mixture comprises an active material and an elastic polymer (apolymer having rubber property).

The present invention further relates to a method for producing apositive electrode plate for an alkaline storage battery comprising thesteps of: controlling the thickness of an original material made of aporous metal having a weight per unit area of 150 to 350 g/m² to form aporous metal substrate; filling a material mixture paste containing anactive material and an elastic polymer into the substrate in a stripepattern to form at least one material mixture paste-filled portion andat least one unfilled portion; drying the substrate filled with thematerial mixture paste; rolling the dried substrate filled with thematerial mixture paste to form an electrode plate; and cutting theelectrode plate into a predetermined size.

The “material mixture paste” used herein is a mixture of a materialmixture and a liquid component (dispersing medium for the materialmixture). The liquid component is removed by the drying step.

The method of the present invention include a method for producing apositive electrode plate for an alkaline storage battery comprising thesteps of: controlling the thickness of an original material made of aporous metal having a weight per unit area of 150 to 350 g/m² to form aporous metal substrate; filling a material mixture paste containing anactive material and an elastic polymer into the substrate in a stripepattern to form at least one unfilled portion where the material mixturepaste is not filled between material mixture paste-filled portions and;drying the substrate filled with the material mixture paste; rolling thedried substrate filled with the material mixture paste to form anelectrode plate; and cutting the electrode plate along the at least oneunfilled portion between the material mixture paste-filled portions intoa predetermined size.

Because the porous metal substrate has a low density with a weight perunit area of 150 to 350 g/m², a high capacity positive electrode can beproduced at a lower cost than using conventional technique. Further,because the material mixture contains an active material and an elasticpolymer, even when the substrate has a low weight per unit area, it ispossible to produce a flexible positive electrode whose currentcollection network is unlikely to be broken. Accordingly, a batteryhaving excellent high rate discharge performance and excellent activematerial utilization rate can be provided. Moreover, because thematerial mixture contains an active material and an elastic polymer,even when the substrate has a low weight per unit area, a positiveelectrode less likely to produce metal burrs and cracks can be obtained.

Note that the “filled portion” used herein means a part of the porousmetal substrate filled with the material mixture or material mixturepaste, and that the “unfilled portion” means a part of the porous metalsubstrate where the material mixture or material mixture paste is notfilled.

The “porous metal substrate” means a substrate made of a metal havingthree-dimensionally connected pores. A preferred example of the porousmetal substrate is a foamed nickel substrate. The foamed nickelsubstrate is produced by, for example, electroplating or electrolessplating a urethane sheet with nickel and then baking the sheet to removea carbon component therefrom. Other than the foamed nickel substrate, asintered substrate or a three-dimensionally structured metal sheet canbe used. The sintered substrate is produced by sintering the powders ofcarbonyl nickel or the like.

The material mixture contains an active material and an elastic polymeras indispensable components. The active material contributes to anelectrochemical reaction, and the elastic polymer functions as a binder.

The elastic polymer has a glass transition temperature (Tg) of less than25° C. (room temperature), and it is a polymer having elasticity at roomtemperature. A preferred example thereof is a copolymer oftetrafluoroethylene and propylene. Particularly preferred is a copolymercomposed of a tetrafluoroethylene (TFE) unit and a propylene (PP) unitat a molar ratio of 30:70 to 70:30. Alternatively, a copolymer composedof a tetrafluoroethylene unit and a propylene unit at a molar ratiosimilar to the above and further a vinylidene fluoride unit in an amountof not greater than 5 mol % can be used.

Preferably, the material mixture further contains at least one selectedfrom the group consisting of xanthan gum, guar gum, carrageenan anddiutan gum. More preferably, the material mixture further containsxanthan gum and carboxymethyl cellulose as a thickener.

As the active material, particles of a nickel oxide such as nickelhydroxide or nickel oxyhydroxide can be used. The particles preferablyhave a mean particle size of 5 to 15, and a BET specific surface area of5 to 15 m²/g. From the viewpoint of increasing high rate dischargeperformance and active material utilization rate, cobalt oxyhydroxidehaving an oxidation number of 2.9 to 3.4 is preferably carried on thesurface of the nickel oxide particles as a conductive material.

From the viewpoint of enhancing the effect of preventing ashort-circuit, the surface of the porous metal substrate is preferablycovered with a layer (hereinafter referred to as surface materialmixture layer) composed of the material mixture having a thickness of 10to 100 μm.

The porous metal that forms the substrate is preferably iron plated withnickel or nickel.

From the viewpoint of obtaining a desired elasticity, the elasticpolymer preferably has a glass transition temperature of −10 to +20° C.The amount of the elastic polymer contained in the material mixture ispreferably 0.2 to 5 parts by weight per 100 parts by weight of theactive material.

The glass transition temperature can be determined by, for example,using a calorimeter, as a temperature at which a change in endothermicrate or thermal expansion coefficient is the greatest when an elasticpolymer is heated. As the calorimeter, a differential scanningcalorimeter (DSC), a thermomechanical analyzer (TMA), etc can be used.

In the production method of the present invention, it is preferred thata compressed gas is sprayed onto a portion of the substrate which willserve as the unfilled portion while the material mixture paste is filledinto the porous metal substrate.

By spraying a compressed gas onto the unfilled portion, the materialmixture that would otherwise spread from the filled portion to theunfilled portion is continuously pushed back to the filled portion.Thereby, the spread of the material mixture paste into the unfilledportion can be prevented, which enables smooth welding between theunfilled portion and a current collector plate.

For example, while the material mixture paste is continuously filledinto the porous metal substrate, a compressed gas is sprayed onto theunfilled portions positioned at the external ends of the filled portion.Alternatively, while the material mixture paste is continuously filledinto the porous metal substrate in a stripe pattern, a compressed gas issprayed onto the unfilled portions each positioned at the external endof each outermost filled portion and the unfilled portion between filledportions.

The step of spraying a compressed gas can be performed efficiently usinga device including, for example, a means to release the hooped porousmetal substrate, a die nozzle having a silt-shaped outlet for sprayingthe material mixture paste, a means to spray a compressed gas, a meansto dry the substrate filled with the material mixture paste, and a meansto wind up the dried substrate filled with the material mixture paste,wherein the die nozzle and the means to spray a compressed gas aredisposed adjacent to each other.

The slit-shaped outlet is an opening in a form of an interstice.Preferably, the space of the interstice is 0.5 to 1.5 mm.

An example of the means to release the hooped porous metal substrate isthe uncoiler 72 of FIG. 9. An example of the means to spray a compressedgas is the compressed gas spraying outlet of FIG. 9. An example of themeans to dry the substrate filled with the material mixture paste is thedrying oven of FIG. 9. An example of the means to wind up the driedsubstrate filled with the material mixture paste is the coiler 79 ofFIG. 9.

In the step of filling the material mixture paste into the porous metalsubstrate, for example, the substrate is allowed to pass in thelongitudinal direction of the substrate through a gap having apredetermined width between a plurality of die nozzles which are facingtowards each other, during which the material mixture paste is sprayedfrom the slit-shaped outlets of the plurality of die nozzles to thesubstrate. The material mixture paste is sprayed onto the substrate in astripe pattern. Thereby, the step of filling the material mixture pasteinto the substrate can be performed efficiently.

When the material mixture paste is filled into the porous metalsubstrate, it is preferred to control the distance between the dienozzle and the substrate or the flow rate of the material mixture pastesprayed from the die nozzle based on the amount of the material mixturepaste filled into the substrate measured by an X-ray weight analyzer orβ-ray weight analyzer and/or the width of the filled portion of thesubstrate measured by an image recognition device.

In the plurality of die nozzles facing towards each other, theslit-shaped outlet is preferably divided into a plurality of sections byat least one partition. By using such die nozzles, a plurality of filledportions in a stripe pattern can be formed when the material mixturepaste is filled in the substrate.

As the plurality of die nozzles facing towards each other, there can beused a combination of a plurality of units, each unit having aslit-shaped outlet for spraying the material mixture paste. In thiscase, the plurality of units are arranged such that their slit-shapedoutlets are aligned in a line. In this case also, a plurality of filledportions in a stripe pattern can be formed when the material mixturepaste is filled in the substrate.

From the viewpoint of stabilizing the paste filling rate in the porousmetal substrate, the plurality of die nozzles are preferably arrangedsuch that the plurality of die nozzles face towards each other with adisplacement of the positions of their slit-shaped outlets in an amountof 1 to 5 mm in a direction in which the substrate passes.

The porous metal substrate is obtained by, for example, by controllingthe thickness of an original material made of a porous metal by pressingor the like. The porous metal substrate preferably has a thickness of200 to 150 μm and a porosity of 88 to 97%. The “porosity” used hereinmeans a volume percentage of the pores (three dimensionally connectedpores) in the substrate.

The volume of the material mixture paste filled in the porous metalsubstrate is preferably 95 to 150% of the volume of the pores in thesubstrate. In other words, the paste filling rate, which is defined by aratio of the volume of the filled paste to the volume of pores in thesubstrate, is preferably 95 to 150%.

When the material mixture paste is applied onto a porous metal substratehaving a low weight per unit area, the material mixture mayunintentionally spread to the unfilled portion by dripping of thematerial mixture paste. In order to prevent the dripping of the materialmixture paste, the material mixture paste preferably has a viscosity at20 rpm of 3 to 15 Pa·s and a viscosity ratio (viscosity at 2rpm/viscosity at 20 rpm) of not less than 2. The viscosity of thematerial mixture paste is measured at 25° C. (room temperature).Further, the material mixture paste preferably has a viscosity at 2 rpmof 10 to 70 Pa·s.

The viscosity at 20 rpm and the viscosity at 2 rpm are measured at arotation speed of 20 rpm and 2 rpm, respectively, at 25° C. by a B typeviscometer.

The material mixture can further contain, in addition to the activematerial and the elastic polymer, the previously mentioned thickenersuch as xanthan gum, guar gum, carrageenan, diutan gum or carboxymethylcellulose. The material mixture can further contain a conductivematerial such as cobalt oxyhydroxide.

In the material mixture paste, the liquid component (dispersing mediumfor the material mixture) is preferably water.

Since a material mixture containing an active material and an elasticpolymer is used with a porous metal substrate having a low weight perunit area in the present invention, it is possible to produce a flexiblepositive electrode plate with a high capacity whose current collectionnetwork is unlikely to be broken and which is less likely to producemetal burrs and cracks at a lower cost than conventional techniques.

In other words, with the use of the positive electrode of the presentinvention, it is possible to produce an alkaline storage battery capableof providing a large discharge capacity even when discharged at a largecurrent and having high active material utilization rate and excellentcharge/discharge cycle characteristics at a lower cost than conventionaltechniques.

Moreover, according to the present invention, the material mixture pastecan be filled into the porous metal substrate efficiently, whichsignificantly reduces the loss of the active material. Further, aprecise definition of the interface between the filled portion and theunfilled portion can be achieved, which contributes to provide a goodwelding condition between the unfilled portion and a current collectorplate.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a front view of a die nozzle used in the present invention.

FIG. 2 is an oblique view of a die coater used in the present invention.

FIG. 3 is a diagram showing the positional relationship between dienozzles and a substrate.

FIG. 4 is a diagram showing an example of the step of cutting a positiveelectrode plate obtained by filling a material mixture paste in asubstrate in a stripe pattern.

FIG. 5 is a front view of a positive electrode cut into a predeterminedsize.

FIG. 6(A) is a cross sectional view of the main part of a positiveelectrode of the present invention.

FIG. 6(B) is a cross sectional view of the main part of a conventionalpositive electrode plate.

FIG. 7 is a graph showing the discharge capacity vs. the number ofcycles for batteries A and B (Graph 1).

FIG. 8 is a graph showing the discharge capacity vs. the number ofcycles for battery packs A and B (Graph 2).

FIG. 9 is an oblique view of another die coater used in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A positive electrode plate for an alkaline storage battery of thepresent invention includes a strip-shaped porous metal substrate and amaterial mixture filled in the substrate. The porous metal substrate hasan unfilled portion where the material mixture is not filled along atleast one of the two longitudinal sides thereof. The important featureof the positive electrode for an alkaline storage battery of the presentinvention lies in that the porous metal substrate has a weight per unitarea of 150 to 350 g/m², and that the material mixture contains anactive material and an elastic polymer.

Because porous metal substrates having a low weight per unit area areproduced at low costs, according to the present invention, theproduction cost for positive electrode can be reduced. Porous metalsubstrates having a low weight per unit area, however, are plagued withproblems that its metal skeleton is easily broken and that its currentcollection network is also easily broken during the production ofpositive electrode plates. For this reason, porous metal substrateshaving a weight per unit area of greater than 350 g/m² haveconventionally been employed.

In contrast, in the present invention, the material mixture contains anelastic polymer, and therefore flexibility is imparted to the positiveelectrode. As such, the problems as described above rarely occur.Accordingly, porous metal substrates having a weight per unit area of150 to 350 g/m² can be utilized effectively. From the viewpoint ofachieving low costs, it is further preferred that the porous metalsubstrate have a weigh per unit area of 190 to 250 g/m².

When the porous metal substrate has a weight per unit area of less than150 g/m², the handling of the substrate will be difficult, and it willbe difficult to prevent the breaking of a current collection network andthe formation of burrs. Further, due to the low strength of thesubstrate, it will be difficult to continuously fill the materialmixture paste into the substrate when in a hoop shape. Conversely, whenthe porous metal substrate has a weight per unit area of above 350 g/m²,the reduction of production costs for positive electrode plates cannotbe achieved. Further, because the substrate will account for a higherpercentage by volume in an electrode plate, the electrode plate willhave a low capacity.

The porous metal substrate is preferably composed of iron plated withnickel or nickel. Particularly preferred is nickel.

When producing the positive electrode using a porous metal substratehaving a low weight per unit area, the formation of burrs is more likelycompared to the case of using conventional substrates. In view of this,from the viewpoint of enhancing the effect to prevent a short-circuit inthe battery due to burrs, the surface of the substrate is preferablycovered with a surface material mixture layer having a thickness of 10to 100 μm. In other words, the substrate preferably carries the materialmixture in an amount exceeding the volume of the pores of the substrate.

The elastic polymer preferably has a glass transition temperature of−100 to +20° C. When the glass transition temperature is less than −100°C., the effect of the elastic polymer to bond the active material willbe small. Conversely, when the glass transition temperature is above+20° C., sufficient flexibility might not be imparted to the positiveelectrode. The addition of an elastic polymer having a glass transitiontemperature of −100 to +20° C. to the material mixture provides amaterial mixture with appropriate flexibility. Accordingly, in therolling step and the cutting step of the electrode plate, the separationof the material mixture can be prevented. Also, the formation of cracksduring the fabrication of the electrode group can be prevented.

Preferred examples of the elastic polymer having the above-describedphysical properties include copolymers of tetrafluoroethylene andpropylene, and copolymers of tetrafluoroethylene, propylene andvinylidene fluoride. Among the latter copolymers, particularly preferredare those having a vinylidene fluoride unit in an amount of less than 5mol % because they have good alkali resistance. Styrene butadiene rubber(SBR) and perfluoroelastomer are also preferably used.

The amount of the elastic polymer contained in the material mixture ispreferably 0.2 to 5 parts by weight per 100 parts by weight of theactive material, more preferably 0.5 to 3 parts by weight. When theamount of the elastic polymer is too large, the dischargecharacteristics and the positive electrode capacity will be decreased.Conversely, when the amount of the elastic polymer is too small, theflexibility of the positive electrode will be reduced, decreasing thecurrent collecting efficiency or facilitating the formation of burrs andcracks on the positive electrode.

From the viewpoint of improving the current collecting efficiency, thematerial mixture preferably contains a conductive material. Examples ofthe conductive material include metal cobalt powders, cobalt hydroxideand cobalt oxyhydroxide. Among them preferred is cobalt oxyhydroxide,more preferably cobalt oxyhydroxide having an oxidation number of 2.9 to3.4. When nickel oxide particles carrying, as the conductive material,cobalt oxyhydroxide having an oxidation number of 2.9 to 3.4 on thesurface thereof are used as the active material, the use of a smallamount of the conductive material efficiently improves the currentcollecting efficiency. The nickel oxide particles carrying cobaltoxyhydroxide on the surface thereof can be produced by treating nickeloxide particles carrying cobalt hydroxide on the surface thereof withheat alkali.

The amount of the conductive material contained in the material mixtureis usually 2 to 15 parts by weight per 100 parts by weight of the activematerial. When cobalt oxyhydroxide is carried on the active materialsurface, the amount of cobalt oxyhydroxide is preferably 3 to 10 partsby weight per 100 parts by weight of the active material.

Hereinafter, a positive electrode plate for an alkaline storage batteryof the present invention will be described with reference to an exampleof the production method therefor.

First, a material mixture paste is prepared by dispersing a materialmixture in a liquid component. The material mixture contains an activematerial and an elastic polymer as indispensable components. Thematerial mixture can further contain a conductive material, a thickener,etc. In particular, a thickener is effective in controlling theviscosity of the material mixture paste.

Preferred examples of the thickener for use include cellulose-basedthickeners such as carboxymethyl cellulose (CMC) and methyl cellulose(MC), and viscosity-enhancing polysaccharides such as xanthan gum, guargum, carrageenan and diutan gum. The amount of the thicnkener containedin the material mixture paste is usually 0.05 to 0.3 parts by weight per100 parts by weight of the active material.

From the viewpoint of imparting appropriate thixotropy to the materialmixture paste, viscosity-enhancing polysaccharides are preferably used.Further, viscosity-enhancing polysaccharides are excellent from theviewpoint of improving the cycle characteristics of alkaline storagebatteries because they are not easily dissolved in aqueous alkalinesolutions compared to cellulose-based thickeners.

In order to fill the paste into the pores of the substrate, the pasteneeds to be fluid to some extent. However, after the paste is filledinto the substrate, the paste should be retained in the position whereit was filled to prevent the paste from dripping. In order to satisfythe requirements, it is preferred to use a paste having high thixotropy,that is, a paste having a low viscosity at high shear and a highviscosity at low shear. Particularly, because the present invention usesa porous metal substrate having a low weight per unit area, it isdesirable that the dripping be prevented by using a paste having highthixotropy.

In the present invention, the thixotropy of the paste is evaluated interms of viscosity ratio: viscosity at 2 rpm/viscosity at 20 rpm. Theviscosity of the paste is measured at 25° C. A paste suitable to befilled into a substrate having a low weight per unit area has aviscosity at 20 rpm of 3 to 15 Pa·s and a viscosity ratio of not lessthan 2. When the viscosity at 20 rpm is less than 3 Pa·s, the drippingmay occur. Conversely, when the viscosity at 20 rpm is above 15 Pa·s, itmay be difficult to fill the paste into the pores of the substrate. Whenthe viscosity ratio is less than 2, the paste may not have a preferredbalance of the filling property and the fluidity. More preferably, theviscosity ratio is not less than 3. There is a limit in increasing theviscosity ratio, and the upper limit is about 7. The paste preferablyhas a viscosity at 2 rpm of 10 to 70 Pa·s.

From the viewpoint of preventing the spread of the material mixturepaste from the filled portion to the unfilled portion, the combined useof the cellulose-based thickener and the viscosity-enhancingpolysaccharide is preferred as the thickener. Particularly, the combineduse of xanthan gum and CMC is preferred.

Xanthan gum is a water soluble polysaccharide, and its aqueous solutionhas a higher viscosity-enhancing capability than CMC aqueous solution.Accordingly, the use of xanthan gum is effective not only in uniformlydispersing the active material in the material mixture paste but also inpreventing the spread of the material mixture paste into the unfilledportion. However, the use of only xanthan gum may result in excessivelyhigh viscosity. For this reason, from the viewpoint of optimizing thefluidity of the material mixture paste, the combined use of xanthan gumand CMC is effective.

When CMC and xanthan gum are used together, the weight ratio between CMCand xangthan gum is preferably CMC:xanthan gum=20:80 to 40:60. The totalamount of CMC and xanthan gum is preferably 0.1 to 0.4 parts by weightper 100 parts by weight of active material.

The porous metal substrate is preferably one having a controlledthickness produced by pressing a hooped original material made of aporous metal having a weight per unit area of 150 to 350 g/m². Thethickness of the porous metal substrate is controlled according to thedesired electrode plate design. The porous metal substrate preferablyhas a thickness of 200 to 1500 μm. When the thickness of the substrateis less than 200 μm, the size of the pores in the substrate will besmall, which might prevent the material mixture paste from entering thepores. Conversely, when the thickness of the substrate is above 1500 μm,in the method of spraying the paste from a die coater, the pressure willbe insufficient and the paste might not be filled into the substrate.

The porosity of the porous metal substrate is preferably controlled to88 to 97%. When the porosity is less than 88%, the permeation of thepaste into the substrate might be decreased. Conversely, when theporosity is above 97%, the substrate will have low strength, making itdifficult to continuously fill the material mixture paste into thesubstrate when in a hooped shape.

A description is now given of an example of the step of applying thematerial mixture paste onto the obtained porous metal substrate.

Preferably, the material mixture paste is applied onto the hooped porousmetal substrate in a stripe pattern to form at least one unfilledportion where the material mixture is not filled. The material mixturepaste having the viscosity and viscosity ratio mentioned previously iseasily filled into the substrate, and it is unlikely to spread into theunfilled portion. The unfilled portion positioned between the filledportions preferably has a width of 1 to 10 mm, but the width is notlimited thereto.

The substrate filled with the material mixture paste is dried. The driedsubstrate filled with the material mixture paste is then rolled to givea hooped electrode plate. The electrode plate is cut into apredetermined size to finally give a positive electrode plate. In thestep of cutting the electrode plate, the electrode plate is cut alongthe unfilled portion, whereby the cut edge serves as an exposed portionof the substrate to which a current collector plate or lead is welded.As such, a step of removing the material mixture having been filled intothe substrate can be omitted. Accordingly, the production method of thepositive electrode plate can be simplified, and the loss of the activematerial can be prevented. Further, the separation of the activematerial can also be prevented by cutting the electrode plate along theunfilled portion.

It is effective to use a die coater as shown in FIGS. 1 and 2 when thematerial mixture paste is filled into the porous metal substrate. Asshown in FIG. 2, at least a pair of die nozzles 21 are placed facingtowards each other with a gap having a predetermined width therebetween.A substrate 22 passes through the gap in the longitudinal directionthereof. The pair of die nozzles 21 facing towards each other spray amaterial mixture paste 23 to the passing substrate 22, during which thematerial mixture paste 23 is filled into the substrate 22 in a stripepattern to form an unfilled portion 24. In order to achieve suchapplication, the die nozzles 21 preferably have a slit-shaped outlet 11as shown in FIG. 1.

The die nozzle 21 of FIG. 1 has a slit-shaped outlet 11 for spraying thematerial mixture paste. The slit-shaped outlet 11 is divided into aplurality of sections by a partition 12. The number of the partition 12is not limited to one. Two partitions or more may be arranged accordingto the required number of unfilled portions. Because the sectionshielded by the partition does not spray the material mixture paste, anunfilled portion is formed on the portion of the substrate correspondingto the partition. As a result, the material mixture paste is applied ina stripe pattern.

The die nozzle may be composed of a combination of a plurality of units,each unit having a slit-shaped outlet for spraying the material mixturepaste. In this case, the plurality of units are arranged such that theirslit-shaped outlets are aligned in a line.

In the case of using a plurality of die nozzles 21 facing towards eachother with a gap having a predetermined width therebetween as shown inFIG. 2, it is preferred that, as shown in FIG. 3, the slit-shapedoutlets 31 for spraying the material mixture paste be arranged such thatthe slit-shaped outlets 31 are displaced from each other by a distanceof 1 to 5 mm (the width “b” in FIG. 3) in the direction in which thesubstrate passes. By arranging the facing die nozzles 21 such that theyare displaced from each other by a distance of 1 mm or more, as thepaste enters the pores in the substrate 22, the air trapped in the poresgradually moves to a portion not filled with the paste. Thereby, it ispossible to bring the paste filling rate of the substrate 22 close tothe most dense state. The distance (the width “a” in FIG. 3) between thesubstrate surface and the nozzle tip (the slit-shaped outlet) ispreferably 10 to 500 μm.

If the slit-shaped outlets 31, each positioned at each side of thesubstrate, are opposed such that they are in exact agreement with eachother, when the paste enters the pores in the substrate 22simultaneously from both sides of the substrate 22, air might be left inthe center portion of the substrate in the thickness direction.Conversely, when the amount of the displacement (the width “b” in FIG.3) is 5 mm or more, the substrate might be curved in the thicknessdirection, causing a variation in thickness of the material mixturelayer.

When the material mixture paste is filled into a porous metal substratehaving pores therein, the width of the material mixture paste actuallyfilled into the substrate (the width of the filled portion) variesrelative to the width of the slit-shaped outlet of the die nozzle. Inother words, the actual width of the filled portion is not always thesame as the width of the slit-shaped outlet of the die nozzle. Suchdifference is caused by the variation in the viscosity of the paste andthe wettability of the substrate. The width of the unfilled portionvaries according to the variation of the width of the filled portion,causing a variation in capacity and size of the resulting positiveelectrode plate.

From the viewpoint of stabilizing the variation in width of the filledportion, it is preferred that the correlations among the pasteviscosity, the paste spraying rate, the pressure around the slit-shapedoutlet, the distance between the die nozzle and the substrate, the pastefilling rate and the width of the filled portion be examined in advance,the result of which is then fed back to the application step. As apreferred example, the amount of the paste filled in the substrate andthe width of the filled portion are monitored. Based on the informationobtained from the monitoring, the distance between the die nozzle andthe substrate or the flow rate of the material mixture paste sprayedfrom the die nozzle is controlled. The amount of the material mixturepaste filled in the substrate can be monitored with the use of an X-rayweight analyzer or β-ray weight analyzer. The width of the filledportion can be monitored with the use of an image recognition device.

The volume of the material mixture paste filled in the substrate ispreferably 95 to 150% of the volume of the pores in the substrate, morepreferably 100 to 130%. By adjusting this amount (paste filling rate) to95% or more, when an electrode plate 40 is cut along the cutting lines41 and 42 as shown in FIG. 4, the area of the substrate metal exposed atthe cross section formed along the cutting line 41 can be reduced. Afterthe cutting step, a positive electrode plate 50 as shown in FIG. 5 canbe obtained. The positive electrode plate 50 has an unfilled portion 51formed along one side of the two longitudinal sides thereof. Theunfilled portion 51 corresponds to the area cut along the cutting line42. When the paste filling rate is above 100%, because a surfacematerial mixture layer 61 is formed on the electrode plate as shown inFIG. 6(A), the exposure of the porous metal 62 at the electrode surfacecan be prevented. As shown in FIG. 6(B), in a conventional positiveelectrode plate, the material mixture does not appear on the substratesurface. In the present invention, however, because the separation ofthe material mixture can be prevented by the elastic polymer, it ispossible to produce an electrode plate having the structure shown inFIG. 6(A).

The formation of the surface material mixture layer on the electrodeplate can prevent cracks or short-circuiting that occurs when thepositive electrode and a negative electrode are spirally wound with aseparator interposed therebetween. As a result, the separator can bemade thinner, and a significant increase in battery capacity can beachieved. However, when the filling rate is above 150%, the thickness ofthe material mixture might be nonuniform due to dripping of the materialmixture paste, or the current collecting efficiency might be decreased.

A description is now given of an embodiment in which a compressed gas issprayed onto a portion of the substrate corresponding to the unfilledportion while the material mixture paste is filled into the porous metalsubstrate.

As described previously, the filling step can be performed moreefficiently by spraying a compressed gas onto the unfilled portionspositioned at the external ends of the filled portion(s) and/or theunfilled portion positioned between filled portions when the materialmixture paste is continuously filled into the porous metal substrate.

Since the material mixture paste has appropriate fluidity so that it canbe filled into the substrate easily, the material mixture paste tends tospread to the unfilled portion immediately after the paste is filledinto the substrate. By spraying a compressed gas onto the unfilledportion, the material mixture paste that would otherwise spread from thefilled portion to the unfilled portion can be pushed back to the filledportion.

When the pressure of the compressed gas is too low, the effect ofpreventing the material mixture paste from spreading will be small.Conversely, when the pressure is too high, the material mixture pastewill be spattered. Accordingly, the pressure of the compressed gas ispreferably controlled to 0.01 to 0.30 MPa.

As for the direction from which the compressed gas is sprayed, thecompressed gas is preferably sprayed from the direction at an angle of 0to 30° with respect to the plane (reference plane) perpendicular to thesubstrate surface, the plane including the interface between the filledportion and the unfilled portion. The spread of the material mixturepaste can be prevented efficiently by spraying the compressed gas fromthe direction at the unfilled portion side at an angle of 0 to 30° withrespect to the reference plane.

The compressed gas for use is a gas that does not poison the materialmixture paste. Specifically, compressed air, high pressure nitrogen orhigh pressure helium can be used.

As for the device for achieving the filling step described above, adevice as shown in FIG. 9 is preferred.

A porous metal substrate 71 in a hoop shape is released from an uncoiler72, and then introduced into a gap between a pair of die nozzles 73. Thepair of die nozzles 73 each have a slit-shaped outlet for spraying amaterial mixture paste, and they are arranged facing towards each otherwith the gap having a predetermined width therebetween. A substrate 71passes through the gap at a predetermined speed, during which a materialmixture paste 74 sprayed from the slit-shaped outlets of the die nozzles73 disposed at both sides is filled into the substrate 71. Compressedgas spraying outlets 76 are disposed at both ends of the slit-shapedoutlet of each die nozzle 73. A compressed gas is sprayed from thecompressed gas spraying outlets 76 onto the unfilled portions 77(exposed portions of the substrate). The compressed gas prevents thematerial mixture paste from spreading from the filled portion 75 to theunfilled portions 77. The substrate filled with the material mixturepaste is then introduced into a drying oven 78. The dried substrate isfinally wound up by a coiler 79.

Although, in the device of FIG. 9, only one filled portion 75 is formed,it is also possible to form a plurality of filled portions 75 on thesubstrate in a stripe pattern by, for example, alternately disposing aplurality of compressed gas spraying outlets 76 and a plurality of dienozzles 73 in parallel with each other and using a porous metalsubstrate having a greater width.

Hereinafter, the examples of the present invention will be described.

EXAMPLE 1

(i) Active Material Carrying Conductive Material on the Surface Thereof

Particles of nickel hydroxide solid solution serving as an activematerial were prepared by the following known method. Specifically, anaqueous solution of sodium hydroxide was added dropwise to anotheraqueous solution dissolving nickel sulfate mainly with predeterminedamounts of cobalt sulfate and zinc sulfate therein while adjusting thepH of the aqueous solution with an aqueous ammonia, whereby sphericalparticles of nickel hydroxide solid solution were deposited.

The obtained particles of nickel hydroxide solid solution were thenwashed with water and dried. The resulting particles, which hereinaftermay be referred to as core particle, had a mean particle size of 10 μmand a specific surface area of 12 m²/g. Note that the mean particle sizewas measured by a laser diffraction particle size analyzer and thespecific surface area was measured by BET method.

Subsequently, fine particles of cobalt hydroxide serving as a conductivematerial were carried onto the particles of nickel hydroxide solidsolution (core particle) by the following known method. Specifically,the particles of nickel hydroxide solid solution and an aqueous solutionof cobalt sulfate (1 mol/L) were slowly added with stirring to anaqueous solution of sodium hydroxide while adjusting the pH of theaqueous solution to be 12 at 35° C. Thereby, fine particles of cobalthydroxide (β type) were deposited on the surface of the particles ofnickel hydroxide solid solution. The resulting particles had a meanparticle size of 10 μm and a specific surface area of was 12 m²/g. Notethat the mean particle size was obtained from an SEM image and thespecific surface area was measured by BET method.

The particles of nickel hydroxide solid solution carrying fine particlesof cobalt hydroxide on the surface thereof were held in a treatmentvessel, to which an aqueous alkaline solution of 45 wt % concentrationwas added at a rate of 0.07 L/Kg and mixed. Hot air at a temperature of100° C. was then sent thereto at a rate of 4 L/min/Kg for drying.Thereby, the cobalt hydroxide on the surface was converted into highlyconductive cobalt oxyhydroxide having an average cobalt valence of 3.1.

(ii) Material Mixture Paste

The particles of nickel hydroxide solid solution carrying cobaltoxyhydroxide (conductive material) on the surface thereof were used asan active material. The amount of the conductive material was 10 partsby weight per 100 parts by weight of the active material.

As a binder, an elastic polymer having a glass transition temperature of−3° C. was used. The elastic polymer was a copolymer containing atetrafluoroethylene unit and a propylene unit at a molar ratio of 55:45with a density of 1.55 g/cm³. The elastic polymer was used as an aqueousdispersion containing 35 wt % elastic polymer. This aqueous dispersionis available from Asahi Glass Co, Ltd. under the trade name of AFLAS.

As a thickener, carboxymethyl cellulose (CMC) and xanthan gum were used.Carboxymethyl cellulose (CMC) was used as 1 wt % aqueous solution.

A material mixture paste was prepared by the following procedure withthe use of the above-prepared materials.

First, 100 parts by weight of the active material carrying theconductive material thereon and 0.2 parts by weight of xanthan gum wereintroduced into a kneader, which was then thoroughly mixed by mixingblades. Five parts by weight of CMC aqueous solution was slowly addeddropwise to the kneader while mixing, and 3 parts by weight of theelastic polymer was further added. Thereby, there was prepared amaterial mixture paste containing the active material carrying theconductive material and the elastic polymer at a weight ratio of 100:3with a water content of 17 wt %.

The resultant material mixture paste had a viscosity of 25 Pa·s at 2 rpmand 5 Pa·s at 20 rpm. The viscosity ratio (viscosity at 2 rpm/viscosityat 20 rpm) was 5. A paste like this has a decreased viscosity when theshear rate y is increased so that the material mixture paste is sprayedfrom the slit-shaped outlet of the die nozzle very smoothly. Further,when the substrate filled with the paste is dried, shear force is notapplied to the paste and the viscosity of the paste is increased, thepaste does not drip. In other words, the paste has very suitablerheology for the filling step using a die coater.

(iii) Positive Electrode Plate

The above-produced material mixture paste was filled into a 160 mm widehooped porous metal substrate made of nickel in a stripe pattern suchthat one unfilled portion where the material mixture was not filled wasformed in the center of the porous metal substrate. The porous metalsubstrate used here was produced by plating a foamed urethane sheet withnickel, followed by baking at 600° C. to remove urethane, after which,the resultant original material of porous metal was pressed to have athickness of 700 μm. The porous metal substrate had a weight per unitarea of 200 g/m² and a porosity of 97%.

In the step of filling the material mixture paste into the substrate,from a pair of die nozzles arranged facing towards each other with a gaphaving a predetermined width therebetween, the material mixture pastewas sprayed in a stripe pattern onto the substrate passing through thegap in the longitudinal direction. The pair of die nozzles were arrangedsuch that the slit-shaped outlets for spraying the material mixturepaste were displaced from each other by a distance of 0.5 μm in thedirection in which the substrate passed. The volume of the materialmixture paste filled into the substrate was adjusted to 130% of the porevolume of the substrate (a paste filling rate of 130%).

The slit-shaped outlets of the die nozzles for spraying the materialmixture paste each had a width of 148 mm. The portion 68 to 80 mm infrom one edge of the slit-shaped outlet was shielded by a partitionhaving a width of 12 mm. Because the portion shielded by the partitiondid not spray the paste, an unfilled portion having a width of 12 mm wasformed in the center of the substrate.

During the filling of the material mixture paste into the substrate, thewidth of the filled portions was monitored by a camera. According to avariation in the width of the filled portion, the distance between thefacing die nozzles was automatically adjusted. The distance between eachof the die nozzles and the substrate was appropriately adjusted withinthe range of 10 to 500 μm.

The substrate filled with the material mixture paste was then dried withhot air at 110° C. for 5 minutes. The dried substrate filled with thematerial mixture was rolled by a roll press to have a thickness of 500μm to give an electrode plate in a hoop shape. The obtained electrodeplate was cut along at least the unfilled portions as shown in FIG. 4.As a result, a positive electrode plate A as shown in FIG. 5 having anunfilled portion where the material mixture was not filled along oneside of the two longitudinal sides thereof was obtained. The main partof the positive electrode plate A had a cross section as shown in FIG.6(A). The unfilled portion was folded twice to increase strength becausea current collector plate would later be welded thereto.

COMPARATIVE EXAMPLE 1

(i) Active Material Carrying Conductive Material on the Surface Thereof

An active material carrying a conductive material on the surface thereofwas produced in the same manner as in Example 1 except for thefollowing.

Particles of nickel hydroxide solid solution carrying fine particles ofcobalt hydroxide were held in a treatment vessel, to which an aqueousalkaline solution of 45 wt % concentration was added at a rate of 0.05L/Kg and mixed. Hot air at a temperature of 60° C. was then sent theretoat a rate of 1 L/min/Kg for drying. Thereby, the cobalt hydroxide on thesurface was converted into cobalt oxyhydroxide having an average cobaltvalence of 2.8.

(ii) Material Mixture Paste

As an active material, the above-produced particles of nickel hydroxidesolid solution carrying cobalt oxyhydroxide (conductive material) on thesurface thereof were used. The amount of the conductive material was 10parts by weight per 100 parts by weight of the active material.

As a binder, polytetrafluoroethylene (PTFE) was used. An aqueousdispersion containing 60 wt % PTFE was prepared.

As a thickener, carboxymethyl cellulose (CMC) was used. An aqueoussolution containing 1 wt % carboxymethyl cellulose (CMC) was prepared.

Using the above materials, a material mixture paste was prepared by thefollowing procedure.

First, 100 parts by weight of the active material carrying theconductive material thereon was introduced into a kneader and thoroughlymixed by mixing blades, during which 2.5 parts by weight of water and 20parts by weight of CMC aqueous solution were slowly added dropwise tothe kneader, and 2 parts by weight of PTFE was further added. Thereby,there was prepared a material mixture paste containing the activematerial carrying the conductive material and PTFE at a weight ratio of100:2 with a water content of 19 wt %.

The resultant material mixture paste had a viscosity of 5 Pa·s at 2 rpmand 2 Pa·s at 20 rpm. The viscosity ratio (viscosity at 2 rpm/viscosityat 20 rpm) was 2.5.

(iii) Positive Electrode Plate

The above-prepared material mixture paste was filled into a 180 mm widehooped porous metal substrate made of nickel. The porous metal substrateused here was produced by plating a foamed urethane sheet with nickel,followed by baking at 600° C. to remove urethane. The porous metalsubstrate had a thickness of 1000 μm, a weight per unit area of 400 g/m²and a porosity of 95%.

In the step of filling the material mixture paste into the substrate,the porous metal substrate in a hoop shape was continuously fed into avessel holding the material mixture paste therein. In this method, thepaste needs to be permeated into the substrate by immersing the hoopedporous metal substrate in the vessel holding the material mixture paste.For this reason, the viscosity of the paste was reduced to theabove-mentioned level (i.e., 5 Pa·s at 2 rpm and 2 Pa·s at 20 rpm).

After the material mixture paste was filled into the substrate, thesurface of the substrate filled with the material mixture paste wassmoothened by a roll smoother. Subsequently, the active material wasremoved by applying an ultrasonic wave to a predetermined portion of theelectrode plate so as to form an unfilled portion similar to theelectrode plate of Example 1 to which a current collector plate would bewelded.

The substrate filled with the material mixture paste was dried with hotair at 110° C. for 15 minutes. The dried electrode plate was rolled by aroll press to have a thickness of 500 μm. The obtained electrode platewas then cut to give a positive electrode plate B as shown in FIG. 5.The main part of the positive electrode plate B had a cross section asshown in FIG. 6(B). The paste filling rate was 90%. The unfilled portionwas folded twice to increase strength because a current collector platewould later be welded thereto.

[Evaluation of Electrode Plate]

(Condition of Positive Electrode Plate)

The positive electrode plates A and B were subjected to the followingevaluation tests. The results are shown in Table 1.

Evaluation test 1: The maximum height of metal burrs formed duringcutting and the number of the burrs were determined from SEM images.

Evaluation test 2: One thousand electrode groups, each electrode groupproduced by spirally winding the positive electrode plate and awell-known hydrogen storage alloy electrode with a 100 μm thickpolypropylene separator therebetween, were produced. During theproduction of each electrode group, the rate (%) of the weight of thematerial mixture separated during the spirally winding step to theweight of the positive electrode plate was determined in percentage. Theaverage of 1000 electrode groups was then calculated, which was referredto as separation rate of material mixture.

Evaluation test 3: One thousand electrode groups, each electrode groupproduced by spirally winding the positive electrode plate and awell-known hydrogen storage alloy electrode with a 100 μm thickpolypropylene separator therebetween, were produced. The defective ratedue to short-circuiting was determined.

Evaluation test 4: The rate (%) of the weight of the material mixturewasted (e.g., the material mixture separated by an ultrasonic wave inthe production of the positive electrode plate B) to the weight of thematerial mixture used in the production of the positive electrode platewas determined in percentage, which was referred to as material mixtureloss rate.

TABLE 1 Positive electrode plate A Positive electrode plate B Eval. 1Maximum height of Maximum height of burrs: 40 μm burrs: 150 μm Number ofburrs: 1 Number of burrs: 10 Eval. 2 Separation rate of materialSeparation rate of material mixture: 0.01% or less mixture: 0.20% Eval.3 Defective rate due to short- Defective rate due to short- circuiting:0% (0 article) circuiting: 2.5% (5 articles) Eval. 4 Material mixtureloss rate: 0% Material mixture loss rate: 15%

As is evident from Table 1, in the positive electrode plate A having apaste filling rate of 130% and using a highly-binding and flexiblebinder with a low glass transition temperature, the number of burrsformed during the cutting step was extremely small compared to thepositive electrode plate B, and the separation rate of the materialmixture during the spirally winding step was also very small.Accordingly, even when a thin separator with a thickness of 100 μm wasused in the positive electrode plate A, no defective electrode group dueto short-circuiting was observed.

(Production of Alkaline Storage Battery)

Using the positive electrode plates A and B, FSC sized nickel-metalhydride storage batteries having a nominal capacity of 3300 mAh wereproduced. Specifically, the positive electrode plate and a negativeelectrode plate were spirally wound with a 100 μm thickhydrophilic-treated polypropylene separator interposed therebetween toform an electrode group. A current collector plate was welded to theporous metal (unfilled portion) of the electrode group exposed at theedge. The electrode group was then housed in a battery case. Thenegative electrode used here was a well-known hydrogen storage alloyelectrode. A specified amount of alkaline electrolyte containingpotassium hydroxide as the main solute dissolved therein at aconcentration of 7 to 8 N was injected to the battery case. The openingof the battery case was then sealed, and the initial charge/dischargewas performed. Hereinafter, a battery produced using the positiveelectrode plate A is referred to as battery A, and a battery producedusing the positive electrode plate B is referred to as battery B.

(Active Material Utilization Rate)

As the initial charge/discharge, each battery was subjected to repeated(twice) charge/discharge cycles in which charging was performed at acharge rate of 0.1 C (1 C=3300 mA) for 15 hours and then discharging wasperformed at a discharge rate of 0.2 C for 6 hours. Subsequently, aging(the activation of the negative electrode alloy) was performed at 45° C.for 3 days, after which the active material utilization rate of thepositive electrode plate was measured by changing the charge/dischargeconditions. The results are shown in Table 2.

TABLE 2 Active material utilization rate 0.2 C 1 C 2 C Positiveelectrode plate A 102% 95% 92% Positive electrode plate B 100% 90% 80%

The active material utilization rate shown in Table 2 is a rate ofdischarge capacity to theoretical capacity of the positive electrode ofeach battery expressed in percentage. The theoretical capacity of thepositive electrode was calculated by multiplying the weight of nickelhydroxide in the positive electrode active material by an electriccapacity of 289 mAh/g assuming that nickel hydroxide in the positiveelectrode active material undergoes one electron reaction.

The discharge capacity was measured by overcharging the battery at acharge rate shown in Table 2 and then discharging the same at adischarge rate of 0.2 C, 1 C and 2 C to a battery voltage of 0.8 V.Table 2 clearly indicates that the active material utilization rate ofthe battery A produced using the positive electrode plate A of thepresent invention was of a higher standard than that of the battery Bproduced using the positive electrode plate B of Comparative Example 1.

(Charge/Discharge Cycle Characteristics)

The charge/discharge cycle characteristics of the batteries A and B wereinvestigated.

A charge/discharge cycle was performed as follows. Charging wasperformed by −ΔV (ΔV=0.01 V) control method at a charge rate of 1 C,after which discharging was performed at a discharge rate of 1 C to abattery voltage of 0.8 V. After every predetermined cycles, charging wasperformed by −ΔV (ΔV=0.01 V) control method at a charge rate of 1 C,after which discharging was performed at a discharge current of 10 A toa battery voltage of 0.4 V. The discharge capacity was measured at thistime. FIG. 7 is a graph (referred to as Graph 1) showing the correlationbetween the discharge capacity measured at this time and the number ofcycles. As is obvious from Graph 1, the battery A achieved a highercapacity than the battery B, and the capacity decrease after long-termcycle life test was suppressed.

Further, using the batteries A and B, battery packs, each battery packincluding 10 batteries (which hereinafter may be referred to as unitcells) connected in series, were produced. The charge/discharge cyclecharacteristics of the battery packs were investigated.

A charge/discharge cycle was performed as follows. Charging wasperformed by ΔT (ΔT=3.0° C./min) control method at a charge rate of 10A, after which auxiliary charging was performed by ΔT (ΔT=3.0° C./min)control method at a charge rate of 5 A. Subsequently, discharging wasperformed at a discharge current of 20 A to a pack voltage of 4 V. FIG.8 is a graph (referred to as Graph 2) showing the correlation betweenthe discharge capacity at this time and the number of cycles.

As is obvious from Graph 2, the battery pack A achieved a highercapacity than the battery pack B, and the capacity decrease afterlong-term cycle life test was suppressed. This tendency was moreremarkable in the charge/discharge cycle characteristic test using thebattery pack than using the single battery. This is because, since thereis a variation in capacity among the unit cells contained in the batterypack, some unit cells are over-discharged when the battery pack issubjected to the charge/discharge cycle.

Once the battery is over-discharged, the capacity decreases because ofthe separation of the active material due to the generation of gas fromthe positive electrode, the melting of the separator due to thegeneration of heat, the decrease of the electrolyte by the operation ofa safety valve and the increase of internal resistance.

On the other hand, according to the present invention, the activematerial is rarely separated in the spirally winding step to constructan electrode group and few burrs and cracks are formed in the electrodeplate, compared to a conventional technique. Accordingly,short-circuiting between the positive and negative electrodes due to themelting of the separator and the capacity decrease due to the separationof the active material can be effectively prevented.

EXAMPLE 2

Positive electrode plates were produced in the same manner as in Example1 except that the paste filling rate was changed to those shown in Table3.

[Evaluation]

The above-produced positive electrode plates were subjected to thefollowing measurement. The results are shown in Table 3.

<i> The maximum height of metal burrs formed during the cutting step wasdetermined.

<ii> One thousand electrode groups, each electrode group produced byspirally winding the positive electrode plate and a well-known hydrogenstorage alloy electrode with a 100 μm thick polypropylene separatortherebetween, were produced. During the production of each electrodegroup, the rate (%) of the weight of the material mixture separatedduring the spirally winding step to the weight of the positive electrodeplate was determined in percentage. The average of 1000 electrode groupswas then calculated, which was referred to as separation rate ofmaterial mixture.

<iii> The thickness was measured at arbitrary 3 points of the electrodeplate before the cutting step. Then, the difference between the minimumthickness and the maximum thickness was determined.

<iv> The thickness range of the surface material mixture layer (i.e. thelayer made of the material mixture covering the substrate surface) wasmeasured.

TABLE 3 Paste filling rate 80% 90% 95% 120% 130% 150% 170% Maximumheight of 150 μm  80 μm  50 μm  30 μm  30 μm  10 μm  10 μm burrsSeparation rate of 0.20% 0.10% 0.01% 0% 0% 0% 0.20% material mixtureThickness  0.6 mm 0.3 mm 0.2 mm 0.2 mm 0.2 mm 0.5 mm 0.8 mm differenceof electrode plate Thickness range of Metal Metal 0-50 10-80 10-10040-110 0-200 surface material substrate substrate mixture layer (μm)exposed exposed

Table 3 illustrates that, when the paste filling rate was 95% or more,metal burrs became small, and the separation rate of the active materialand the variation in thickness also became small. As can be seen, thereis a tendency that the higher the paste filling rate, the lower theseparation rate of the active material. This is presumably due to theeffect of the elastic polymer. However, when the paste filling rate wasabove 150%, the amount of the separated material mixture increased, andthe variation in thickness of the electrode plate also increased.

EXAMPLE 3

Positive electrode plates were produced in the same manner as in Example1 except that the amount of CMC per 100 parts by weight of the activematerial carrying the conductive material and the amount of xanthan gumper 100 parts by weight of the same were changed to those shown in Table4 and that the paste viscosity and the viscosity ratio were changed tothose shown in Table 4. Then, nickel-metal hydride storage batterieswere produced in the same manner as in Example 1 except that theabove-produced positive electrode plates were used.

[Evaluation]

The positive electrode plates and the nickel-metal hydride storagebatteries produced above were subjected to the following measurement.The results are shown in Table 4.

<i> The variation of the paste filling rate was determined. Here, thepaste filling amount was monitored by an X-ray weight analyzer duringthe step of filling the paste to determine the maximum amount and theminimum amount. Then, the difference between them was calculated. Thepercentage of the calculated value to the specification value(theoretical value of the paste filling rate determined from theporosity of the substrate) was referred to as variation of paste fillingrate.

<ii> The active material utilization rate of the electrode plates wasdetermined in the same manner as used for the battery A of Example 1.

TABLE 4 Xanthan gum 0.02 0.06 0.08 0.04 0.10 0.15 0.17 0.18 (part byweight) CMC 0.18 0.14 0.12 0.16 0.10 0.05 0.03 0.02 (part by weight)  2rpm 5 8 12 12 15 25 38 80 (Pa · s) 20 rpm 2 3 3 8 6 8 10 12 (Pa · s)Viscosity ratio 2.5 2.7 4 1.5 2.5 3.1 3.8 6.7 (2 rpm/20 rpm) Variationof  9%  7%  5%  7%  5%  5%  5%  7% paste filling rate Active material99% 100% 102% 99% 102% 102% 102% 100% ulitization rate

Table 4 illustrates that, when the material mixture paste had aviscosity at 20 rpm of 3 to 15 Pa·s and a viscosity ratio of 2 or more,the variation of paste filling rate was small and good active materialutilization rate was obtained. It is also clear from Table 4 that thepreferred viscosity at 2 rpm is 10 to 70 Pa·s. When the material mixturepaste had a viscosity and a viscosity ratio which were out of the range,the variation of paste filling rate was relatively large and the activematerial utilization rate was relatively low. The reason for this ispresumably because the material mixture paste was not sufficientlypermeated into the substrate or because even if the material mixturepaste was sufficiently permeated into the substrate, the materialmixture paste dripped due to its low viscosity.

EXAMPLE 4

Positive electrode plates were produced in the same manner as in Example1 except that the amount of the binder (elastic polymer) contained inthe material mixture per 100 parts by weight of the active material waschanged to those shown in Table 5. Then, nickel-metal hydride storagebatteries were produced in the same manner as in Example 1 except thatthe above-produced positive electrode plates were used.

[Evaluation]

The above-produced positive electrode plates and nickel-metal hydridestorage batteries were subjected to the following measurement. Theresults are shown in Table 5.

<i> One thousand electrode groups, each electrode group produced byspirally winding the positive electrode plate and a well-known hydrogenstorage alloy electrode with a 100 μm thick polypropylene separatortherebetween, were produced. During the production of each electrodegroup, the rate (%) of the weight of the material mixture separatedduring the spirally winding step to the weight of the positive electrodeplate was determined in percentage. The average of 1000 electrode groupswas then calculated, which was referred to as separation rate ofmaterial mixture.

<ii> One thousand electrode groups, each electrode group produced byspirally winding the positive electrode plate and a well-known hydrogenstorage alloy electrode with a 100 μm thick polypropylene separatortherebetween, were produced. The defective rate due to short-circuitingwas determined.

<iii> The active material utilization rate of the positive electrodeplates was determined in the same manner as used for the battery A ofExample 1.

TABLE 5 Binder amount 0 0.1 0.2 1 2 3 5 6 (part by weight) Separationrate of 0.16% 0.12% 0.05%  0%  0%  0%  0%  0% material mixture Defectiverate due to   5%   3%  0.7%  0.3%  0%  0%  0%  0% short-circuitingActive material   97%   99%  101% 102% 102% 102% 101% 98% utilizationrate

Table 5 illustrates that batteries having a low defective rate due toshort-circuiting and an excellent active material utilization rate wereobtained when the amount of binder was 0.2 to 5 parts by weight per 100parts by weight of the active material, preferably 1 to 5 parts byweight. As can be seen, the defective rate due to short-circuiting tendsto be lower with an increased amount of the binder. This is presumablybecause the flexibility of the positive electrode plate was increasedand the formation of cracks or burrs was prevented. It is also clearfrom Table 5 that the separation of the active material does not easilyoccur when the amount of the binder is increased. However, when theamount of the binder was above 5 parts by weight per 100 parts by weightof the active material, the active material utilization rate decreased.Accordingly, the amount of the binder is preferably 5 parts by weight orless.

REFERENCE EXAMPLE 1

(i) Material Mixture Paste

A material mixture paste with a water content of 20 wt % was prepared bymixing 100 parts by weight of nickel hydroxide, 10 parts by weight ofcobalt oxide, 0.2 parts by weight of CMC as a thickener, 0.3 parts byweight of PTFE as a binder and an appropriate amount of water.

The nickel hydroxide had a mean particle size of 10 μm and a specificsurface area of 10 m²/g. The cobalt oxide had a mean particle size of0.3 μm and a specific surface area of 20 m²/g. Note that the meanparticle size was determined by a laser diffraction particle sizeanalyzer, and the specific surface area was measured by BET method.

The obtained material mixture paste had a viscosity of 40 Pa·s at 2 rpmand 10 Pa·s at 20 rpm. The viscosity ratio (viscosity at 2 rpm/viscosityat 20 rpm) was 4.

(ii) Positive Electrode Plate

The above-prepared material mixture paste was filled into a porous metalsubstrate made of nickel in a hoop shape using a device as shown in FIG.9. The porous metal substrate used here was a substrate having a widthof 80 mm, a thickness of 1.5 mm (1500 μm), a weight per unit area of 350g/m² and three-dimensionally connected pores with a mean pore size of200 μm.

The porous metal substrate in a hoop shape was released from anuncoiler, and then introduced into a 1.65 mm gap between a pair of dienozzles arranged facing towards each other from below to above. Thematerial mixture paste was sprayed from the slit-shaped outlets of thepair of die nozzles and filled into the substrate. Compressed gasspraying outlets were arranged at both ends of the slit-shaped outlet ofeach of the die nozzles. Compressed air was sprayed therefrom. Theslit-shaped outlets had a width of 60 mm, and thus the filled portionhad a width of 60 mm. On the external ends of the filled portion wereformed unfilled portions having a width of 10 mm. The paste filling ratewas 110%.

The compressed air sprayed onto the external ends of the filled portionhad a pressure of 0.05 MPa. The compressed air was sprayed from adirection perpendicular to the substrate surface. The substrate filledwith the material mixture paste was then introduced into a drying ovenand dried at 120° C. at 5 minutes, after which the dried substrate waswound up by a coiler and then rolled to have a thickness of 0.7 mm. Theobtained electrode plate in a hoop shape was cut into a predeterminedsize to give a positive electrode plate for an alkaline storage battery.

REFERENCE EXAMPLE 2

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 1 except that 0.05 parts byweight of CMC per 100 parts by weight of nickel hydroxide and 0.15 partsby weight of xanthan gum per 100 parts by weight of the same were usedas the thickener.

REFERENCE EXAMPLE 3

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 1 except that a punched ironsheet plated with nickel having a thickness of 80 μm, a pore size of 1.5mm and a porosity of 40% in a hoop shape was used as the substrate.

REFERENCE EXAMPLE 4

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 1 except that the width ofthe porous metal substrate was changed to 160 mm, that two pairs of dienozzles arranged in parallel were used, that compressed gas sprayingoutlets were disposed between the two pairs of die nozzles and theexternal ends of the two pairs of die nozzles, and that filled portionseach having a width of 60 mm was formed in a stripe pattern. Theunfilled portion formed between the filled portions had a width of 20mm.

REFERENCE EXAMPLE 5

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 4 except that 0.05 parts byweight of CMC per 100 parts by weight of nickel hydroxide and 0.15 partsby weight of xanthan gum per 100 parts by weight of the same were usedas the thickener.

REFERENCE EXAMPLE 6

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 4 except that a punched ironsheet plated with nickel having a thickness of 160 μm, a pore size of1.5 mm and a porosity of 40% in a hoop shape was used as the substrate.

REFERENCE EXAMPLE 7

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 1 except that the spraying ofthe compressed air was not performed.

REFERENCE EXAMPLE 8

A positive electrode plate for an alkaline storage battery was producedin the same manner as in Reference Example 3 except that the spraying ofthe compressed air was not performed.

[Evaluation]

The positive electrodes of Reference Examples produced above weresubjected to the following evaluation. The results are shown in Table 6.

(Width Variation of Filled Portion)

The width of the filled portion was measured at 50 points uniformlyspaced on the filled portion. A standard deviation a for width variationwas determined. In the measurement, a part of the unfilled portion intowhich the material mixture paste spread was regarded as filled portion.

(Weld Defect Inspection)

One hundred cylindrical electrode groups, each electrode group producedby spirally winding the positive electrode plate and a well-knownhydrogen storage alloy electrode with a separator made of sulfonatedpolypropylene non-woven fabric therebetween, were produced. A currentcollector plate was welded to the unfilled portion of the positiveelectrode plate at an end of the electrode group, after which a checkwas made to see if the part of the separator around the welded portionwas discolored. The number of defective electrode groups having adiscolored separator was counted.

TABLE 6 Width Spraying variation Number of of Number of of filledelectrode compressed filled portion group with gas portions ThickenerSubstrate σ (mm) weld defect Ref. Ex. 1 Yes 1 CMC Porous 0.18 1 metalRef. Ex. 2 Yes 1 CMC + xanthan Porous 0.13 0 gum metal Ref. Ex. 3 Yes 1CMC Punched 0.25 2 metal Ref. Ex. 4 Yes 2 CMC Porous 0.19 1 metal Ref.Ex. 5 Yes 2 CMC + xanthan Porous 0.15 0 gum metal Ref. Ex. 6 Yes 2 CMCPunched 0.27 2 metal Ref. Ex. 7 No 1 CMC Porous 0.45 5 metal Ref. Ex. 8No 1 CMC Punched 0.55 5 metal

Table 6 illustrates that, compared to Reference Examples 7 and 8 inwhich compressed air was not sprayed to the unfilled portions, inReference Examples 1 to 6, the width variation of the filled portion wasreduced and the number of electrode groups having a weld defect wassignificantly decreased. Noteworthy among them are Reference Examples 2and 5 in which CMC and xanthan gum were used as the thickener. InReference Examples 2 and 5, no weld defect was observed because thespread of the material mixture paste was successfully prevented.

The present invention is broadly applicable to positive electrodes foralkaline storage batteries using a porous metal substrate havingthree-dimensionally connected pores. According to the present invention,it is possible to provide an alkaline storage battery capable ofproviding a large discharge capacity even when discharged at a largecurrent and having superior high rate discharge characteristics, highactive material utilization rate and excellent charge/discharge cyclecharacteristics at a lower cost than conventional techniques. Alkalinestorage batteries to which the present invention is applicable includenickel-metal hydride storage batteries which are used as the powersources for portable devices, power tools, hybrid electric vehicles(HEV), etc, and nickel-cadmium storage batteries, etc.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A positive electrode plate for an alkaline storage battery comprisinga strip-shaped porous metal substrate and a material mixture filled insaid substrate, wherein: said substrate has an unfilled portion wheresaid material mixture is not filled along at least one of twolongitudinal sides of said substrate, said substrate has a weight perunit area of 150 to 350 g/m², said material mixture comprises an activematerial and an elastic polymer, said elastic polymer including acopolymer of tetrafluoroethylene and propylene, or a copolymer oftetrafluoroethylene, propylene and vinylidene fluoride, and having aglass transition temperature of −100 to +20° C., and said materialmixture further comprises xanthan gum and carboxymethyl cellulose. 2.The positive electrode plate for an alkaline storage battery inaccordance with claim 1, wherein said active material comprises nickeloxide particles, and said nickel oxide particles carry, as a conductivematerial, cobalt oxyhydroxide having an oxidation number of 2.9 to 3.4on the surface of said nickel oxide particles.
 3. The positive electrodeplate for an alkaline storage battery in accordance with claim 1,wherein a surface of said substrate is covered with a layer comprisingsaid material mixture and having a thickness of 10 to 100 μm.
 4. Thepositive electrode plate for an alkaline storage battery in accordancewith claim 1, wherein a porous metal that forms said substrate is madeof iron plated with nickel or made of nickel.
 5. The positive electrodeplate for an alkaline storage battery in accordance with claim 1,wherein: the amount of said elastic polymer contained in said materialmixture is 0.2 to 5 parts by weight per 100 parts by weight of saidactive material.
 6. The positive electrode plate for an alkaline storagebattery in accordance with claim 1, wherein said carboxymethyl celluloseand said xanthan gum are contained in said material mixture at a weightratio of 20:80 to 40:60, carboxymethyl cellulose to xanthan gum.
 7. Thepositive electrode plate for an alkaline storage battery in accordancewith claim 1, wherein said xanthan gum and said carboxymethyl celluloseare contained in said material mixture in a total amount of 0.1 to 0.4parts by weight of said xanthan gum and said carboxymethyl cellulose per100 parts by weight of said active material.